Electrophotographic, computer controlled printers have become pervasive in the office, factory, print shop, copy center, and home. An electrophotographic printer operates by transferring toner to plain paper and fusing the toner by means of heat, pressure, and/or other fixing technologies. The pattern of the transferred toner may form characters, graphic images, etc.
The term electrophotography refers to the use of modulated light, frequently a scanned laser beam, to create an electrostatic latent image on a photoconductive carrying medium such as a drum or belt. The latent electrostatic image is formed by momentary electrical conductivity of the photoconductor in response to exposure to the modulated light. The momentary conductivity allows a surface charge to discharge through the photoconductor to a conductor held at a bias voltage at locations corresponding to the modulated light exposure.
FIG. 1 is a diagram illustrating the principal features of an electrophotographic printer. A photoconductive drum 102 is rotated past a charging or sensitization station 104 that deposits a static charge substantially uniformly over the surface of the drum 102. An imaging module 106 modulates light selectively over the surface of the photoconductor 102. This causes the static charge in those spots receiving light to discharge through the photoconductive layer to a conductive layer on the backside of the photoconductor surface. The pattern of discharged and non-discharged spots is referred to as a latent electrostatic image or latent image.
Electrophotographic printers may be made to write-white or write-black. In a write-black system, the toner charge is selected to be attracted to the photoconductor backside conductive layer bias voltage and repelled from the sensitization static charge deposited on the photoconductor surface. Thus, the spots “written” by the modulated light correspond to black areas of the printed page.
Once the electrostatic latent image is formed, the photoconductor 102 is further rotated to a developer 108, where oppositely charged toner, most often in the form of fine, dry particles, is attracted to and deposited on the surface of the photoconductor in a pattern corresponding to the latent image. The photoconductor 102 is further rotated to a transfer point, where the patterned toner is then transferred to the paper 112, often using an electrostatic attraction element 110 such as a corona wire in the form of a corotron or scorotron.
The paper 112, with toner loosely adhered thereto, is fed forward through a fusing station 114 that, generally through a combination of heat and pressure, causes the thermoplastic toner particles to permanently adhere to the paper, thus forming a robust image.
Following transfer of the toner, the photoconductive medium 102 is rotated past a discharge lamp 116 and a cleaner 118, and then repeats the process as it is rotated to the sensitizer or charger 104.
In various printers, light emitting diode (LED), liquid crystal shutter (LCS), vacuum fluorescent, and other types of arrayed light modulator write heads have been used for modulating light onto the photoconductor. Generally though, scanned laser beam exposure or imaging modules have gained favor in the art due to an appropriate balance of cost, speed, performance, and durability. An electrophotographic printer that uses a scanned laser beam to provide light modulation onto the surface of the photoconductive medium may be conveniently referred to as a laser beam printer or LBP.
FIG. 2 illustrates the general construction of an LBP exposure unit 106 made according to the prior art with a rotating polygon beam scanner. A laser diode 202 having a wavelength matched to the sensitivity of the photoconductor (often infrared in the case of an organic photoconductor) is modulated with an image signal. Beam-forming optics 204 produce a laser beam having a desired shape and trajectory. The laser beam is reflected off a rotating polygon mirror 206 and is scanned across the photoconductor 102 through optical elements 208. It may be noted that the design of the exposure module 106 is such that the reflective facets 210a, 210b, etc. of the rotating polygon 206 are placed forward of the center of rotation such that the arriving beam sweeps over each mirror surface as it is deflected across its deflection angle, the deflection angle being sufficient to traverse the photoconductor 102.
One difficulty encountered with scanned laser beam exposure modules relates to the technology used to scan the laser beam. Most frequently, rotating polygon mirrors have been used. Rotating polygon mirrors may suffer from relatively large mass, slow ramp-up to speed, large size, noise, bearing reliability issues, relatively high power consumption, and other shortcomings.